In order to determine the orientation of a carrier, in particular in order to navigate an airplane, it is known that an inertial device can be used that comprises one or more axially symmetrical vibrating sensors, each making use of a resonator possessing symmetry of order equal to or greater than 4 and implementing two degenerate modes of vibration that are identical and orthogonal, having an elastic line that possesses periodicity of order n relative to the axis of symmetry, i.e. an elastic line having n times the same vibration pattern over one revolution around the axis of symmetry. In particular, it is known to use sensors of order 2 such as hemispherical bell vibrating sensors including an appropriate number of electrodes, or quapasons for which the vibration orientations of the two modes are not geometrically orthogonal but are modally orthogonal, i.e. modally offset by π.
It is also known that an axially symmetrical vibrating sensor is adapted to operate in free gyro mode or in rate gyro mode. In free gyro mode the vibration is sustained, but its position is left free; when the orientation of the carrier varies, the position of the vibration relative to its base is representative of the angle through which the carrier has turned.
In rate gyro mode, the vibration is sustained and its position relative to its base is kept constant by delivering suitable electronic commands; the values of said commands are then representative of the speed or rate of rotation of the carrier in inertial space.
It is also known that vibrating gyros present a drift error that is manifested when the gyro is operating in free gyro mode by variation in the position of the vibration even when the carrier is not subject to any rotation. This drift has two components, a constant component, which for a hemispherical resonant gyro is of the order of a few hundredths of a degree per hour, and a component known as alternating drift that is made up of harmonics, mainly a harmonic having the same order n as the vibrating sensor and a harmonic of order twice that of the vibrating sensor, depending on the position of the vibration. With a hemispherical resonator gyro, the harmonic of order n gives rise to a drift error of the order of one degree of per hour and the harmonic of the order 2n gives rise to a drift error of the order of one-tenth of a degree per hour.
It is known to calibrate gyros in a workshop and to draw up correction tables that can be applied while the gyro is in use. Nevertheless, drift error is not only a function of the position of the vibration, but also a function of ambient temperature and of the aging of the gyro. In practice, possibilities for correction are therefore limited.